High visible transmittance, low emissivity coatings applied to transparent substrates such as glass are characterized by their ability to transmit visible light while minimizing the transmittance of other wavelengths of light, such as light in the infrared spectrum. This characteristic is particularly useful for reducing radiative heat transfer without impairing visibility, such as in architectural glass or automobile windows. For aesthetic reasons, in many such applications it is important to maintain reflection relatively consistent throughout the visible spectrum so that the coating has a "neutral" color, i.e., is essentially colorless.
Generally speaking, such high transmittance, low emissivity coatings comprise a "film stack" having at least one thin metallic film or layer with high infrared reflectance and low transmissivity disposed between metal oxide layers. The metallic layer may be virtually any reflective metal, such as silver, copper or gold, with silver being the most frequently-used metal for this application due to its relatively neutral color. Metal oxides useful in high transmittance, low emissivity films including oxides of titanium, hafnium, zirconium, niobium, zinc, bismuth, indium and tin. Prior art systems have also employed oxides of metal alloys, such as zinc stannate or oxides of indium/tin alloys.
The metal oxide layers of such coatings serve two important functions. First, they serve to reduce the visible reflection of the film stack, enhancing transmittance. The metal oxides used in these layers should have a relatively high index of refraction, e.g., on the order of 2.0 or more, in order to achieve this end. According to commonly accepted principles in the art, the metal oxide layer between the transparent (e.g., glass) substrate and the first metallic layer must be at least 300 Angstroms (A) in order to obtain a neutral, high transmitting coating; such layers more commonly are between about 400 and 700 A in thickness.
Second, the metal oxide layers should serve to protect the reflective metal layer from the environment. Once such coated substrates are assembled and installed for use, the film stack is frequently isolated from contact with the environment, as by disposing the film stack between two spaced panes of glass in a composite window structure. However, before these products are assembled, the film stack is frequently subjected to relatively harsh conditions, such as by handling, shipping or washing.
A variety of attempts have been made to enhance the ability of the metal oxide layers to protect the reflective metal layer in such film stacks. For instance, Gillery, et al. teach the use of titanium oxide as a protective overcoat in U.S. Pat. No. 4,786,563, the teachings of which are incorporated herein by reference. Although Gillery, et al. explain that titanium oxides achieve the best results, they note that such an overcoat could be formed of a metal instead of a metal oxide; titanium and alloys of iron or nickel are listed as prime candidates for such a metal layer. Gillery, et al. also teach that certain other oxides simply lack the requisite durability to be used as a protective overcoat. Zinc oxide, bismuth oxide and tin oxide are all listed as having undesirable properties, such as a lack of durability, which make them unsuitable for a protective overcoat.
However, it has been found that the use of a titanium oxide overcoat such as that taught by Gillery, et al. is particularly prone to scratching or abrasion during shipping and washing operations. For instance, when coated substrates are washed before being assembled into a final product, the film stack comes into physical contact with a washing apparatus. It has been found that such a washing stage can physically abrade an overcoat of titanium oxide or the like, noticeably degrading the appearance of the finished article.
The particular composition of the layers of a film stack, as well as their relative thicknesses, must be carefully chosen in order to achieve the desired properties in the coated substrate. As noted above, in addition to maximizing visible transmittance while minimizing emissivity, in many instances it is desirable that a film stack have a neutral color. It has been possible to provide coatings for substrates which have an acceptable neutral color in transmission when viewed at an angle of incidence generally perpendicular to the plane of the film. However, as the angle of incidence is decreased, the film stack will tend to exhibit increasing color in transmission. It has been noted that film stacks which are nearly completely neutral when viewed at a perpendicular angle will tend to exhibit a distinct, visible color when viewed at a more acute angle. Windows of buildings or cars are viewed from various angles, of course, so variations in color of film stacks can frequently be detected in use.
It would be desirable to provide a film stack which serves as a high visible transmittance, low emissivity coating for a substrate, yet remains substantially neutral at a wide range of angles of incidence. Additionally, it would be desirable to provide the film stack with a protective coating which can withstand the rigors of normal handling associated with such substrates.